The answer is B, but the C is also a part it plays in keeping it from dropping too fast.
What Is The Main Purpose Of The Counter Weight Of A Railroad-crossing Barricade?
I believe that B and C may be a part of it but also there is the possibility that a vehicle may be under the bar when it lowers and by adding a counter weight, it would reduce the damage to the vehicle when the bar comes down and the vehicle is under the bar.
To prevent impatient motorists